We are analyzing the biochemical function of the lexA repressor. We have purified the protein and shown that it represses several E. coli genes. We will purify and characterize mutant forms of the repressor, looking for specific changes in the affinities for one or more operators. We will also isolate operator mutants which have reduced affinity for repressor, and characterize them in vitro and in vivo. We will also investigate further the kinetics of derepression in vivo in an effort to characterize more fully the SOS response to treatments which damage DNA.